in the number of simultaneous atrial reentrant wavelets.11,12 Cox et
al. subsequently mapped multiple reentrant atrial wavefronts during
human AF, and this formed the basis for the surgical maze procedure
in which multiple, small, electrically-isolated atrial compartments
were created to prevent sustained reentry.13
More recently it has become accepted that separate mechanisms
may be responsible for triggering and sustaining AF. Focal discharges
(especially from within the pulmonary veins, as described by
Haissaguerre et al)14 can initiate AF. However, AF maintenance probably
involves some form of reentrant activity, with the observed irregular
fibrillatory activity caused by ‘wavebreak’ of the main reentrant
wavefront into multiple chaotic daughter wavelets as a consequence
of inhomogeneity in atrial structure, refractoriness and conduction
velocity.15 Additionally, the mechanisms that sustain AF may evolve
over time as the atria electrically and structurally remodel and AF
progresses from paroxysmal, to persistent and then permanent forms.
This concept has been supported by multiple studies which have
demonstrated more frequent reentrant drivers of AF in patients with
longstanding arrhythmia.

Figure 1: Schematics of Anatomic and Functional
Reentrant Circuits
A

B

Scar

Refractory
Tissue

Wavefront

Wavefront
Wavetail

Excitable gap
Wavetail

No Excitable Gap

A. A simple reentrant circuit around an anatomic barrier (scar). The wavefront is
represented by the blue arrow, and the wavetail is represented by the end of blue shading.
The size of the circuit’s excitable gap is shown between the wavefront and wavetail in
white.
B: Leading circle reentry. There is no excitable gap as the wavefront continuously
encroaching on the wavetail. Because of constant centripetal activation of the center of the
circuit, this area becomes refractory and unexcitable which allows reentry to sustain itself in
the absence of an anatomic barrier.

Figure 2: Leading Circle Reentry

Functional Reentry and the Leading
Circle Model
Functional reentry in its simplest form can be described by the
‘leading circle model’, first described by Allessie et al. in 1977.16 In
this model, circus movement of a unidirectional wavefront results
in constant centripetal activation of the centre of the circuit which
renders it continuously refractory. This refractory area then forms a
functional barrier which can sustain reentry in a way similar to a fixed
anatomic barrier such as a scar (see Figure 1). In the leading circle
model, unidirectional block in tissue allows an impulse to initiate
circus movement in one direction, with the impulse simultaneously
spreading radially outwards to activate the adjacent myocardium and
radially inwards towards the centroid of the circuit.
The wavelength of the circuit – defined as the product of the impulse
conduction velocity and the tissue refractory period – describes
the distance traveled by the wavefront during the refractory period.
Wavelength is critical to understanding how reentry is established
in this model. Consider a prototypical circular reentrant circuit
with a circumference or path-length equal to the wavelength of
the circuit (see Figure 2A). This circuit will have no excitable gap
and will rotate continuously with the leading edge of the impulse
(the wavefront) encroaching on tissue which has just recovered
excitability (the wavetail). This will therefore define the smallest
circuit which can sustain reentry. A smaller circuit, with path length
less than wavelength, would occur in areas located closer towards
the centroid of the circuit (see Figure 2C). This smaller circuit will
not be able to sustain reentry because the circulating wavefront
will encounter refractory tissue and will therefore block and terminate.
A larger circuit, with path length greater than wavelength, would occur
in areas located radially further away from the centroid of the circle
(see Figure 2B) and can sustain reentry with an excitable gap. However,
if conduction velocity throughout the atria remains relatively fixed,
this larger reentrant circuit will rotate and activate the surrounding
myocardium at a slower frequency.
The ‘leading circle’ reentrant circuit with its path length/circumference
equal to its wavelength (see Figure 2A) is thus the smallest circuit
that can sustain reentry. By virtue of its smallest size it also rotates

A: Leading circle reentry. No excitable gap is present, and the circuit rotates with a
frequency of f0.This is the smallest circuit which can sustain functional reentry.
B: Functional reentrant circuit larger than the leading circle. An excitable gap is present and
the circuit rotates with a frequency of f1 which is slower than f0.
C: Functional reentrant circuit smaller than the leading circle. This circuit cannot sustain
reentry as the circulating wavefront encounteres refractory tissue and will therefore block
and terminate. See text for additional details.

with the highest frequency, and so will ‘overdrive’ and suppress all
larger circuits while maintaining a core of refractory tissue towards